The U.S. Environmental Protection Agency (EPA) recently proposed changes to the air quality modeling guidelines to replace the ISCST3 model with the AERMOD model for applications where building downwash is not important, and to replace ISCST3 with ISC-PRIME for applications where downwash is important. In response to public comments, EPA has incorporated the PRIME building downwash algorithms into the AERMOD model. This paper describes the development and developmental evaluation of the AERMOD-PRIME model. The development of AERMOD-PRIME was guided by the goal of keeping the PRIME downwash algorithms as intact as possible, while incorporating the improved meteorology of the AERMOD model. The developmental evaluation of AERMOD-PRIME includes graphical and statistical comparisons of AERMOD-PRIME to ISC-PRIME for one long-term SO2 database and three short-term tracer databases.